1. Field of the Invention
The present invention relates to a method of producing a wiring board having a three-dimensional structure. Further, the present invention relates to an electronic circuit module for which a three-dimensional wiring board produced by employing the foregoing method is used.
2. Description of the Related Art
A wiring board having a three-dimensional structure with a predetermined number of conductor circuits arranged on the surface of a molded product (molded article) molded of an electrical insulating resin so as to improve a space utilization efficiency, and moreover, serve not only as a mechanical component effective for the purpose of mechanical protection from undesirable breakage or damage but also as a circuit wiring component has been heretofore put in practical use.
To improve performances of the three-dimensional wiring board such as a wiring density and others, conductor circuits are arranged on the opposite surfaces of the wiring board such that they are electrically connected to each other.
A conventional method of producing a three-dimensional wiring board of the aforementioned type has been heretofore practiced in the following manner.
First, an electrical conductive metallic material layer is formed on the surface of a molded product obtained by molding an electrical insulating resin while having a perforated three-dimensional substrate structure by way of steps of chemical plating (non-electrolytic plating) and electrical plating (electrolytic plating). Subsequently, an etching resist is printed on the electrical conductive metallic material which in turn is subjected to exposing treatment and developing treatment, and thereafter, unnecessary parts are removed by employing an etching process, from the regions where conductor circuits are formed. Finally, the remaining etching resist is removed away from the surface of the molded product, whereby production of a three-dimensional wiring board is completed.
However, the conventional method as mentioned above has such a problem that many complicated treatment steps comprising a step of roughing the surface of the molded product, an activating treatment step, a sensitizing treatment step, a chemical plating step and an electrical plating step are required for forming conductor circuits on the surface of the molded product obtained by molding an electrical insulating resin.
In addition, since the surface of the molded product obtained by molding an electrical insulating resin is built in the three-dimensional configuration, it is difficult to uniformly coat the surface of the molded product with an etching resist, and moreover, it is also difficult to produce a mask for effectively masking the three-dimensional surface of the molded product during exposing treatment. For this reason, conductor circuits can be arranged only on a comparatively flat part of the surface of the molded product. Another problem of the conventional method is such that chemical roughing treatment to be conducted for bringing a plated film in close contact with the surface of the molded product at the preceding stage of the chemical plating process requires means differing from resin to resin, and moreover, the kind of a resin for enabling the surface of the molded product to be roughed is imitatively restricted only to an ABS resin, a polysulfon resin, a polyacetal resin or the like.
In view of the aforementioned problems, in contrast with the conventional method of forming conductor circuits directly on the surface of a molded product obtained by molding an electrical insulating resin by way of a chemical plating step and others, a proposal has been made with respect to a method of reversely securing conductor circuits to the surface of a molded product molded by employing an injection molding process.
For example, a method of producing a three-dimensional wiring board wherein the method is practiced by way of a step of adhesively forming a film of an electrical insulating inorganic material having a reverse pattern relative to a required conductor circuit on the molding surface of at least one of a upper die half and lower die half constituting an injection molding die, a step of electrically plating the molding surface of the die half of the injection molding die having the film of an electrical insulating inorganic material adhesively formed thereon with a reverse pattern relative to the required conductor circuit to adhesively form the required conductor circuit on the molding surface of the injection molding die and a step of assembling the die half having a conductor circuit adhesively formed therein with the other molding die half of the injection molding die so as to allow a predetermined resin to be injected into the assembled injection molding die while reversely securing the conductor circuit to the surface of a molded product has been disclosed (see official gazettes of Japanese Unexamined Publication Patent (Kokai) No. Hei 3-78289 and Japanese Unexamined Publication Patent (Kokai) No. Hei 3-78291). With respect to these methods described above, however, no consideration has been taken on the formation of a three-dimensional wiring board having a multi-layered structure and an electronic circuit module including three-dimensionally wired conductor circuits for processing signals at a high speed.
In practice, there are many cases where electronic components are assembled on conductor circuits arranged on a multi-layered wiring board like a three-dimensional wiring board to construct an electronic circuit module including three-dimensionally wired conductor circuits.
With respect to the electronic circuit module as mentioned above, it is often required that a gas having a lowest specific dielectric constant is used as an electrical insulating layer in order to process signals at a high speed. Generally, an organic or inorganic high molecular weight material is used for the three-dimensional wiring board as an electrical insulating material to be interposed between adjacent conductor circuits. However, in view of the fact that signals are processed at a high speed in an electronic circuit module having a three-dimensional wiring board of the foregoing kind used therefor in proportion to 1/.sqroot..epsilon.(.epsilon.e: fraction of the specific dielectric constant of electrical insulating layer) inherent to the electrical insulating layer, many requests have been raised from users for a method of three-dimensionally arranging conductor circuits while a gas having a lowest specific dielectric constant is used as an electrical insulating layer.